The following description relates to an exemplary situation which may occur when a terminal or a user equipment (UE) moves from one eNode-B (also called one base station) currently receiving a service to another eNode-B. If an eNode-B managing the UE is changed to another eNode-B, a handover from the eNode-Bs is carried out.
The handover occurs when a radio frequency (RF) channel currently used by a mobile station subscriber is in poor condition, or also occurs when the UE moves from a current eNode-B area to another eNode-B area.
The handover may be classified into a softer handover, a soft handover, and a hard handover according to channel switching methods.
The softer handover is carried out in one cell, and may indicate that a channel used by the UE is switched to a good channel from among channels of a UE's cell coverage. The soft handover indicates that two neighboring channels are simultaneously operated and finally one of the two neighboring channels is slowly severed. According to Code Division Multiple Access (CDMA) scheme, two or more eNode-Bs use the same frequency band, such that this soft handover can be easily implemented in the CDMA scheme. According to the hard handover, a current call-connected channel is severed, and is immediately connected to another channel.
According to the entity carrying out the above-mentioned handover, the handover may be classified into a Network Controlled Handoff (NCHO), a Mobile Assisted Handoff (MAHO), a Mobile Controlled Handoff (MCHO), etc.
The soft handover and the MAHO from among the above-mentioned schemes will hereinafter be described in detail. Needless to say, another handover method may also be used.
An Intra Radio Access Technology (I-RAT) handover supports a handover between homogeneous networks. If a UE moves from one eNode-B (i.e., a serving eNode-B) to another eNode-B (i.e., a target eNode-B) within the range of a homogeneous network, the I-RAT handover provides this UE with a seamless handover and service continuity, resulting in greater convenience of a user of the UE.
A Packet Data Convergence Protocol (PDCP) is an example of broadband mobile communication technologies, and is used as one of Universal Mobile Telecommunication System (UMTS) traffic layers. A wireless PDCP compresses header information of IP-based data streams (e.g., TCP/IP or DTP/UDP/IP-based data streams), such that it may increase the efficiency of data transmission. Also, the PDCP maintains and manages a sequence number (SN) of a radio bearer, such that it transmits user data and implements lossless SRNS re-allocation.
The PDCP layer is defined in a packet-switched area, and is located on a user plane. If the PDCP entity of the transmission end receives a PDCP Service Data Unit (PDCP SDU) from an upper layer, packet header information is compressed by a unique header compression technique, such that a PDCP Protocol Data Unit (PDCP PDU) is constructed. Individual PDCP PDUs are sequentially transmitted to a Radio Link Control (RLC) layer of a reception end in the order of receiving the individual PDCP PDUs from the upper layer, and the RLC layer may transmit data to an RLC layer of the reception end during a proper operation mode. The PDCP entity of the reception end receives the PDCP PDU from the RLC layer, and then recovers an original PDCP SDU.
However, if the UE carries out the handover, unexpected problems may occur when data is transmitted via the PDCP layer. For example, while the UE is handed over from one eNode-B to another eNode-B, a serving gateway (S-GW) may transmit user data to the UE. For example, if the UE is handed over from the serving eNode-B (S-eNB) to the target eNode-B (T-eNB), the order of data received in the target eNode-B (T-eNB) may be inconsistent with the order of other data forwarded from the serving gateway (S-GW).
That is, the user data, which is forwarded from the serving gateway (S-GW) to the UE over a mobile communication network (e.g., a 3GPP LTE network), is transmitted to the UE via the serving eNode-B (S-eNB). In this case, if the UE is handed over to the target eNode-B (T-eNB), the user data, which has been forwarded from the serving gateway (S-GW) to the serving eNode-B (S-eNB), is transmitted to the target eNode-B (T-eNB) after the completion of the handover.
In this case, the order of data received in the target eNode-B (T-eNB) may be unavoidably inconsistent with the order of other data forwarded from the serving gateway (S-GW). Also, if erroneous packet data occurs when data is transmitted and received over the PDCP layer, it is difficult to quickly recover the erroneous packet data.